Attenuation and source properties at the Coso geothermal area, California

We use a multiple-empirical Green's function method to determine source pro perties of small (M - 0.4 to 1.3) earthquakes and P- and S-wave attenuation at the Coso Geothermal Field, California. Source properties of a previousl y identified set of clustered events from the Coso geothermal region are fi rst analyzed using an empirical Green's function (EGF) method. Stress-drop values of at least 0.5-1 MPa are inferred for all of the events; in many ca ses, the corner frequency is outside the usable bandwidth, and the stress d rop can only be constrained as being higher than 3 MPa. P- and S-wave stres s-drop estimates are identical to the resolution limits of the data. These results are indistinguishable from numerous EGF studies of M 2-5 earthquake s, suggesting a similarity in rupture processes that extends to events that are both tiny and induced, providing further support for Byerlee's Law. Wh ole-path Q estimates for P and S waves are determined using the multiple-em pirical Green's function (MEGF) method of Hough (1997), whereby spectra fro m clusters of colocated events ata given station are inverted for a single attenuation parameter, kappa, with source parameters constrained from EGF a nalysis. The kappa estimates, which we infer to be resolved to within 0.01 sec or better, exhibit almost as much scatter as a function of hypocentral distance as do values from previous single-spectrum studies for which much higher uncertainties in individual kappa estimates are expected. The variab ility in kappa estimates determined here therefore suggests real lateral va riability in Q structure. Although the ray-path coverage is too sparse to y ield a complete three-dimensional attenuation tomographic image, we invert the inferred kappa value for three-dimensional structure using a damped lea st-squares method, and the results do reveal significant lateral variabilit y in Q structure. The inferred attenuation variability corresponds to the h eat-flow variations within the geothermal region. A central lowe region cor responds well with the central high-heat flow region; additional detailed s tructure is also suggested.